Thread: Help, please. Second battery for LR

I have a twin battery setup in my Rangie that I did myself. This is how I did it, a battery cable runs from the man battery to a starter solenoid, then from the starter solenoid to the second battery, between the solenoid and the second battery there is an isolator switch so that I can manually disconnect the second battery if I need to, the negative on the second battery simply goes to the body work. There is no point in putting an isolator switch on the negative side. The way I did mine means I can remove the second battery and just let the positive cable sit there. If the isolating switch was on the negative cable I could not do this as the current from the main battery would continue to flow causing a nasty big spark. The starter solenoid is connected to the ignition so when the ignition is on both batteries are connected and both can be charged. When the ignition is off the 2 batteries are no longer connected and the main battery cannot be run down unless I leave the headlights on. I don't have a fuse between the 2 batteries because I don't need one. You could put one in if you like, but you don't have one between you battery and the starter motor and there's no problems with that. Most accessories you connect to your car have a fuse anyway and if they don't it's a simple thing to do. As far as the inverter is concerned, simply hook it up to the second battery or a cigarette lighter socket whichever yours is and leave it at that. So there you go, a bit of a long winded answer, but I hope it helps.

Cheers.

My unqualified opinion

Why fit an breaker switch to the neg side - if there is a fault and neg goes to earth - the result is nothing - no damage - ziltch. If a live goes to ground - shorts, sparks, potential fire.

All wiring in my view should be fused - and the bigger the supply the more important it is - as if you have a small 0.50 mm wire it will melt / burn out - and thus before long break the circuit (bit like a small fuse) whereas id you have a donkin great cable - the amount of damage done before that disintigrates could be huge, and the increased risk of fire is monumental.

have a look at these fuses, and I am sure that somewhere on the internet you can buy them and have them posted.

Cooper Bussmann ® - Heavy Duty Fuses

From what I understand - negative fusing is okay on components motors/lights etc., as the load flows through - but where you have battery to battery - I would really be more comfortable to know something was protecting me.

I may have this totally wrong - and fortunately there are a few on here who have their finger on the pulse a bit more than me - I am only saying what I think is right and what I would be comfortable with.

I'm not an auto electrician so I may be wrong about this, BUT!!!!
Unless your veghicle has a positive earth, then you should not set it up that way. The way I see it, evey accessory you attach to your vehicle has its own earth, so if you put the switch on the earth cable of your isolator, then the positive draw is still going to happen.

Now, I've recently put a second battery in my Disco.
The isolator works so that the primary battery is the only battery in the system till it gets to 13volts, at that point the isolator clicks and the current goes to the secondary battery also. It stays this way till the primary battery drops below 12 vols, then the isolator switches so that you only have 1 battery.
NOW..Ihave a positive lead running from my primary battery to the isolator, then from there to the secondary battery. My primary batery is earthed to the vehicle as is my secondary battery.

For accesories, I have attached a fuse box to my positive lead, from this I run my fridge, DVD etc. (outlet plugs) and as stated above earth each device (or plug) to the chassis.

In this way my vehicles electrical system pretty much works as normal, the accesories work off the accessory battery.

Now this is a very simple system that works for me.
There are some very complicated systems out there BUT!
I would be getting some specific info on the system they're using, and ringing an auto elec BEFORE picking up your vehicle. This doesn't sound right to me.

If you isolate your secondary battery via the earth, then I guess that you're breaking any circuit with that battery anyway????????????????

FAAAARRRKKK!!!!

My head hurts??????????

My head hurts, too; I've been on this all day.

You guys are so helpful. I am about to go back to the thread and digest what you say.

Originally Posted by Disco95

The way I see it, evey accessory you attach to your vehicle has its own earth, so if you put the switch on the earth cable of your isolator, then the positive draw is still going to happen.

Erm, well it depends how its all wired up. You still need a complete loop from +ve terminal, through the device and back to -ve terminal for any current to flow ( that's why it's called a "circuit" )
By "open circuiting" the negative wire from the battery to chassis ( which from the description it seems how it was done in this case ) there's no way current can flow through any device connected to that battery, if that device also has it's negative path through the chassis.

However depending on the type of isolator fitted, and the earthing arrangements, its concievable those devices would then draw current from the main battery instead.

We really need a circuit diagram of the whole setup to tell.

But I don't see any real benefit of putting the switch in the negative wire. I think it's more this guy has probably always done it that way and it's worked OK, so he's not going to change it.

Personally I'm in favour of putting fuses and switches in the positive leads, and running BOTH +ve and -ve wires between the batteries, and between battery and devices. Relying on dubious screwed joints and welds in your bodywork/chassis to carry the negative part of the circuit is always a problem waiting to happen. This is termed an "earth" but it's really a misomer because your vehicle is insulated from Earth by four bloody great rubber tyres ! Each wire running from the battery +ve should have a fuse or circuit breaker installed as close to the battery terminal as possible. Amp rating of the fuses and size of the wire depends on what you are running off that circuit.

Do a search in Projects and Tutorials, there are a number of setups in there.

FWIW, I have a large switch (marine) on which can be selected Bat 1, Bat 2, Both Bat's, or off. For accessories I have a lead from starter motor (whichever battery is selected the starter will draw from) to a fuse block and all power points or accessories draw from there. Most carry a separate earth and others are body earths, and I have had no issues.

I haven't got a scanner to hand, but here's a description of the circuit as it's been wired.

Cable from + terminal of batt #1 to + terminal of batt #2

Cable from neg terminal of Batt #1 to chassis earth

Cable from neg terminal of batt #1 VIA CUT-OFF SWITCH to neg terminal of batt #2

That's it.

Thank you for all comments so far.


By "open circuiting" the negative wire from the battery to chassis ( which from the description it seems how it was done in this case ) there's no way current can flow through any device connected to that battery, if that device also has it's negative path through the chassis.

waynep, did you mean unless that device also has its negative path ............